Sugarcane Biowaste-Derived Biochars as Capacitive Deionization Electrodes for Brackish Water Desalination and Water-Softening Applications

The sugarcane ethanol industry is currently generating an intensive amount of biowaste while consuming significant water resources. In this work, sugarcane bagasse fly ash (SCBFA), a major biowaste with high amounts of fixed carbon, is employed as a precursor for activated carbon (SCBFA-AC) production. Here, SCBFA-ACs are valorized as the main component of carbon electrodes employed in capacitive deionization (CDI), an emerging desalination technology. In this way, an abundant and low-cost biowaste could be used as a green alternative to treat the water. Different activation methods of SCBFA were explored obtaining SCBFA-AC with a broad spectrum of structural and chemical properties. The electrochemical characterization of SCBFA-AC showed the positive impact of large surface areas, good combination of micro- and mesopores, and the presence of surface functional groups on specific capacitances (117 F g^-1). Subsequently, CDI and membrane CDI experiments showed the importance of ion-exchange membranes on improving charge efficiency values (from 5-30 to 80-95%) and consequently, salt adsorption capacity, SAC, from ≈5 to 22 mg g^-1. This SAC value, one of the highest ever obtained with biowaste electrodes, only suffered a slight reduction (19 mg g^-1) after 70 CDI cycles. Finally, SCBFA electrodes were successfully tested for water-softening applications, reaching 15 mg g^-1 when operating using CaCl₂ solutions instead of NaCl. This study represents a great example of the water-energy-food nexus in the framework of the circular economy.